1. Field of the Invention
The present invention relates to a ciphering device for transforming a binary word of "n" bits into another binary word of "n" bits, by the known method of substitutions-permutations. This method consists of carrying out, from the input word, a succession of substitution operations, each consisting of replacing, under the control of a key, a word of "m" bits, (generally subwords of the input word) by another word of "m" bits, and permutations which consist in permuting the bits of one word, following a predefined scheme, so as to form a new word of equal length.
2. Description of the Prior Art
One example of such a ciphering system is formed by the system known as "Data Encryption Standard". A drawback of such a system is that the law of substitution is fixed once and for all, and that all the users of such a system use the same ciphering algorithm, the only parameter variable from one user to another being the key.
The fact that the laws of substitution are fixed and known presents two types of vulnerability, the first vulnerability is that the knowledge of the key is sufficient to decipher a message, the second is that with the substitutions fixed invariable, there is no time limit for finding, by mathematical or statistical methods of analysis, possible weaknesses facilitating deciphering.
Furthermore, enciphering devices existing at the present time are applied to words and keys of well defined length, but lack flexibility for use with words or keys of different lengths.